Durable concrete and method for producing the same

ABSTRACT

A concrete mix for producing freeze-thaw durable concrete having enhanced strength properties, like compressive strength, abrasion resistance, impact strength, toughness, is disclosed. The novel concrete mix contains deformable solid elements in place of 4-8% entrained air for good durability of concrete under freeze-thaw cycles.

CROSS REFERENCE TO RELATED APPLICATION

This application claims benefit under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 61/823,962, filed May 16, 2013.

FIELD OF THE INVENTION

This invention relates to improved concrete and finished articlescomprised of improved concrete. In particular, the invention relates toconcrete mix containing deformable solid elements and the resultingconcrete suitable for freezing environments.

BACKGROUND OF THE INVENTION

Concrete mix is comprised of mainly Portland cement, aggregates (gravel,sand), water and admixtures. A chemical reaction between cement andwater forms hydrated cement paste (referred to as cement gel) that whenhardened binds the aggregates together to give concrete itscharacteristic strength.

The porosity of hydrated cement gel consists of two types of pores: gelpores and capillary pores. For water-to-cement weight ratios at or below36%, the porosity of cement gel is due to the porosity of hydratedcement gel. At water-to-cement weight ratios above 36%, the porosity ofhydrated/hardened cement gel becomes partly due to the cement gel, andpartly space not filled by the cement gel; that which is left behind byunused water upon evaporation, called “capillary pores”. The higher theinitial water-to-cement weight ratio above 36%, the greater thecapillary pores volume.

The cement gel pores are very small (0.0005-0.01 micrometers). Water insuch small pores, below 0.0033 micrometers, cannot freeze down totemperatures as low as −40° C. On the other hand, since capillary poresare much larger (0.02-10 micrometers), the water in these capillarypores freezes quite easily. Water freezing in the capillary poresexpands and if the expansion is not adequately relieved, it can causeexcessive stress to the walls of cement gel pores and cause the brittlecement gel pore walls to break (typical tensile strength of cement gelis about 1000 psi), causing deterioration of the concrete over repeatedfreeze-thaw cycles.

Widely practiced water-to-cement weight ratios in wet concrete mix are40-60% (generally 40-50%) for good workability of wet concrete mix, butwhen the water-to-cement weight ratio is greater than 36%, thesubsequently hardened concrete exhibits poor durability underfreeze-thaw cycles. In order to ameliorate this problem, theconventional practice is to entrain about 4-8% by volume air in the wetconcrete mix, for making the resulting hardened concrete more durableunder freeze-thaw cycles.

Entrained air in a wet concrete mix is in the form of well-dispersed airbubbles and is achieved by adding special chemical admixtures, known asAir Entraining Agents (AEA), in the wet concrete mix. AEAs are generallycomprised of wetting agents, surfactants, and/or foaming agents. Theentrained air bubbles provide space for accommodating expansion of waterfreezing in the capillary pores, thus preventing the walls of the cementgel pores from experiencing excessive stress that could cause cement gelwalls to crack. An entrained air content of about 4-8% by volume in theconventional concrete matrix corresponds to about 16-32% by volume inthe cement gel (assuming the most common value of about 25% cement gelvolume in concrete matrix). This is a huge volume fraction of anon-strength-contributing factor in the concrete matrix (cement gel).

Entrained air bubbles produced by AEAs can have a wide dimensiondistribution and maximum air bubble dimension, such as from 10-1000micrometers (e.g., 90% of air bubbles of dimension above 300micrometers, as reported in “Investigation into Freezing-thawingDurability of Low Permeability Concrete with or without Air EntrainingAgent”, June 2009, National Pavement Concrete Center, Iowa StateUniversity, Ames, Iowa). The entrained air bubbles accommodate expansionof water freezing in capillary pores. Typically, the tensile strength ofa cement gel is about 1000 psi, but water freezing in capillary porescan travel up to 550 micrometers under 1000 psi of pressure generated bythe expansion of the freezing water.

Accordingly, the American Concrete Institute recommends spacing betweenair bubbles (also termed the Spacing Factor) of no more than 200micrometers for entrained air bubbles to be effective in making concretefreeze-thaw durable. In addition, because of the very large dimensiondistribution of entrained air bubbles, including air bubble dimensionsas high as 1000 micrometers, the specific surface area of entrained airbubbles is recommended to be greater than 20 mm²/mm³, in combinationwith a Spacing Factor of 200 micrometers, as recommended by ACI(specific surface area is calculated as total surface area of entrainedair bubbles divided by total volume of entrained air bubbles).

Although these conventional freeze-thaw durable concrete mixes providegood freeze-thaw durable concrete, the entrained air negatively impactsthe compressive strength of concrete in a significant manner, reducingcompressive strength of the concrete by about 5% for every 1% increasein entrained air (reference: US Army Corps of Engineers Report No.ERDC/CRREL TR-02-5, February 2002). The loss in compressive strength ofconcrete due to entrained air is of significant concern in case of highstrength concrete. High strength concrete is often made withsilica-fumes and water-to-cement weight ratios below 40%. Such concretewithout entrained air has shown very high initial compressive strength,but exhibits low durability under freeze-thaw cycles for water-to-cementweight ratios as low as 36%. High strength concrete using silica fumesand water-to-cement weight ratios of about 25% can exhibit very highstrength as well as high freeze-thaw durability. However, it isvirtually impossible to make such low water content concrete mixesconsistently, as outlined in the above referenced US Army Corps ofEngineers Report No. ERDC/CRREL TR-02-5. There are many factors that aredifficult to control affecting air bubble dimension distribution andaverage bubble dimension, for example the nature of the admixtures used,their compatibility with other ingredients in the concrete mix, thetypes of cement and aggregates, water quality parameters like hardness,environmental conditions, etc., which makes air entrained concretenoticeably variable in performance-in-place.

The presence of large air bubbles and a large volume fraction of air inthe cement gel have a pronounced negative synergistic effect on strengthproperties of such concrete. For example, US Army Corps of EngineersReport No. ERDC/CRREL TR-02-5 reports a 5% decrease in compressivestrength of concrete for each 1% increase in entrained air content. Saidsynergistic effect can also reduce other strength-related properties ofhardened concrete, like abrasion resistance, toughness, impact strength,and thus, reduce the overall durability of concrete in use; e.g.,infrastructures, highways.

It is also known in the art to make light weight concretes (LWC), whichare comprised of hollow spheres and formulated to have densitiesgenerally below 1.5 g/cm³, in contrast to most widely used concreteshaving densities above 2 g/cm³. In these materials, low density isachieved by using high volume fractions of low density fillers likehollow spherical elements, generally at least about 25 vol. %(irrespective of whether light weight hollow elements/fillers areorganic, inorganic, polymeric, metallic, hybrid or combinationsthereof).

Although volume fraction is the most important parameter affecting thedensity of LWC, often the amount of hollow elements/fillers in LWC isreported in weight percentage. The weight fraction of hollow elementscan be calculated as follows:Weight fraction=(volume fraction×true density of hollowelement)/(density of LWC)The weight fraction of hollow elements/fillers in LWC for reducingdensity of concrete is generally at least 2%.

Accordingly, there is a need for providing a concrete mix formulationthat can produce wet concrete mixes having good workability, as comparedto the workability of conventional freeze-thaw durable wet concretemixes having 4-8% entrained air, and a resulting hardened concretehaving improved compressive strength and good freeze-thaw durability,again as compared to the compressive strength and the freeze-thawdurability of hardened concrete obtained from conventional freeze-thawdurable wet concrete mixes having 4-8% entrained air.

SUMMARY OF THE INVENTION

In a first embodiment, the present invention is directed to a concretemix for producing a freeze-thaw durable concrete, comprising cement,deformable solid elements, and optionally water, wherein said deformablesolid elements are either included in said concrete mix in a dry state,or mixed with said cement at the time of preparing a wet concrete mix,are present at a level of less than or equal to about 2 vol % relativeto the volume of said wet concrete mix or a green concrete or asubsequently hardened concrete formed therefrom, and have a maximumdimension of less than about 300 micrometers.

Preferably, the deformable solid elements are present at a level of lessthan or equal to about 1 vol %, or even less than or equal to about 0.6vol % of the wet concrete mix or a subsequently hardened concrete formedtherefrom. Likewise, the deformable solid elements have a maximumdimension of less than about 300 micrometers, preferably less than about200 micrometers, or even less than about 100 micrometers, or even lessthan 70 micrometers.

Additionally, the concrete mix can contain sand and/or aggregates andconventional admixtures, including but not limited to one or more ofretarders, accelerators, plasticizers, fillers, silica fumes, fly ash orother pozzolanic materials. However, AEAs are not included within thescope of the admixtures useful in the present invention.

Advantageously, the concrete mix has a total deformable volume of saiddeformable solid elements which is at least 9% of capillary poresvolume, wherein the capillary pores volume is equal to volume of waterin excess of 36% by weight of cement in wet concrete mix.

Accordingly, the total volume of said deformable elements is sufficientto accommodate expansion caused by water freezing in capillary pores ofsaid subsequently hardened concrete during a freezing event.

Conveniently, the concrete mix can be formulated such that water ispresent at a water-to-cement weight ratio in said wet concrete mix of atleast about 26%, or wherein the water-to-cement weight ratio in said wetconcrete mix is at least about 36%, or even wherein the water-to-cementweight ratio in said wet concrete mix is from about 40% to about 50%.

In some embodiments, the deformable solid elements are hollow, and canbe in the form of spheres or fibers, such as wherein said deformablesolid elements are hollow spheres of materials selected from the groupconsisting of glass, ceramics, silica, polymers, and combinationthereof. In a particularly advantageous embodiment, the deformable solidelements are cenospheres.

Alternatively, the deformable solid elements are fibers, such aspolymeric fibers including polypropylene (PP) fibers and polyvinylalcohol (PVA) fibers, and can even act as reinforcing fibers andcontribute to the tensile strength of the green concrete or subsequentlyhardened concrete.

Another embodiment of the present invention is directed to a hardenedconcrete durable to cracking in a freeze-thaw environment, comprisingthe following components: cement, aggregates, admixtures, and deformablesolid elements, wherein the total volume of said deformable solidelements is sufficient to accommodate expansion caused by water freezingin capillary pores of said hardened concrete during a freezing event,and the maximum dimension of said deformable solid elements is less thanabout 300 micrometers, wherein said hardened concrete has a densityabove about 1.5 g/cm³.

According to this embodiment, it is preferable that the deformable solidelements are hollow, such as wherein the deformable solid elements arein the form of spheres or fibers.

Advantageously, the hardened concrete has a density from about 2 g/cm³to about 2.5 g/cm³, or even a density greater than about 2.5 g/cm³.

In another embodiment, the hardened concrete comprises deformable spacefor accommodating expansion caused by water freezing in capillary poresof a cement gel of said concrete during a freezing event, wherein saiddeformable space is less than or equal to about 2 vol % relative to thevolume of the hardened concrete, such as less than or equal to about 1vol %, or even less than or equal to about 0.6 vol % of said concrete.

DETAILED DESCRIPTION OF THE INVENTION

In the present application, the term “concrete” is applicable tocementitious matrix and articles of cementitious matrix in general here,including self-leveling concrete, precast concrete products, concreteproducts produced by various processes like concrete pipes produced byrotational casting, shotcreting/spraying. Also, the term cement refersto “Portland cement” in general.

In contrast to conventional freeze-thaw durable concretes, the concretemix of the present invention is formulated to contain deformable solidelements to replace the void volume of 4-8% entrained air in theconventional wet concrete mixes, with all other concrete mix componentsbeing the same. When combined with water, the concrete mix according tothe present invention offers (1) the same level of workability as aconventional wet concrete mix, and (2) a resulting hardened concretehaving the same level of freeze-thaw durability, and even higher levelsof compressive strength and other strength properties, like abrasionresistance, impact strength, toughness, and hence provides a higherlevel of overall durability as compared to conventional freeze-thawdurable concrete.

According to the present invention, a novel concrete mix for producing afreeze-thaw durable concrete is disclosed, which comprises at leastcement and deformable solid elements, as will be described in moredetail below. The deformable solid elements are either included in saidconcrete mix in a dry state, or mixed with said cement at the time ofpreparing a wet concrete mix, are present at a level of less than orequal to about 2 vol % relative to the volume of said wet concrete mixor a green concrete or a subsequently hardened concrete formedtherefrom, and have a maximum dimension of less than about 300micrometers.

The novel concrete mix can be in the dry form, as described above andpackaged to ship, or can optionally include water to form a wet concretemixture suitable for working and pouring. As such, those skilled in theart will recognize that the novel concrete mix of the present inventioncan be formulated on-site, during mixing of a wet concrete, by adding asuitable amount of deformable solid elements, as described herein.Likewise, the novel concrete mix of the present invention, wet or dry,can further include sand and/or aggregates, such as gravel or the like,and conventional admixtures, such as one or more of retarders,accelerators, plasticizers, fillers, silica fumes, fly ash or otherpozzolanic materials.

Unlike earlier efforts at formulating lightweight, low density concreteby mixing large quantities of hollow spheres with the other componentsof a concrete mix, the volume of deformable solid elements added to aconcrete mixture according to the present invention, herein additionallyreferred-to as the “deformable volume”, is limited to be just sufficientfor accommodating expansion caused by water freezing in capillary poresof a cement gel of said concrete during a freezing event. Thus, the 4-8%entrained air volume of conventional freeze-thaw durable concrete mixesis replaced by deformable solid elements according to the presentinvention. Advantageously, the variability of deformable solid elementdimensions and dimension distributions is selectable and controllable,in contrast to the air bubble dimensions and distributions inconventional freeze-thaw durable concrete. The characteristics of thedeformable solid elements according to the present invention arecontrolled in their manufacturing; e.g., maximum dimension, voidfraction.

The deformable solid elements can be in the form of spheres or fibers,such as wherein said deformable solid elements are hollow spheres ofmaterials selected from the group consisting of glass, ceramics, silica,polymers, and combinations thereof. For example hollow glassmicrospheres, Styles S22, S15, K1, K15, manufactured by 3M™ Corporationof St. Paul, Minn., have desired physical dimensions (90% of hollowspheres having outer diameters less than 65, 90, 105 and 115 micrometersfor Styles S22, S15, K15 and K1 respectively; 50% of hollow spheressmaller than about 60 micrometers; void fraction+90% for above mentionedhollow spheres styles) and low crushing strength of about 250-400 psi(well below 1000 psi tensile strength of cement gel), and thus, are goodcandidates for use in novel concrete mixtures per this invention. StyleS22 can be effective for water-to-cement weight ratios of 40-60% whenused at volume fractions at least equal to or greater than the lowestfeasible for water-to-cement ratios 60%, 50% and 40% (volume fraction ofItems 4A, 10A and 18A respectively), as shown in Table 1A. Said 3M™hollow glass spheres Styles S22, S15, K15 and K1 have about 50% ofspheres smaller than about 60 micrometers and are suitable overwater-to-cement ratios from 40-60% in producing freeze-thaw durableconcrete per this invention.

Another example of solid hollow glass, ceramic spheres having rigidwalls and suitable for use in the novel concrete mixes of this inventionis cenospheres, which are hollow ceramic particles; by-products of coalburning power plants.

Alternatively, when in the form of fibers, they can be hollow fibers,wherein the fibers have aspect ratios of at least about 5, reasonabletensile strength and suitable surface area for enhancing mechanicaland/or chemical anchoring to surrounding cement gel, so that thedeformable fibers can contribute to tensile strength-related productattributes of the concrete matrix (including green concrete; a wetconcrete mix during early stages of curing), in addition to making theresulting hardened concrete freeze-thaw durable.

The deformable solid hollow elements useful in the present invention canbe comprised of organic materials, polymeric materials, for examplepolyvinyl alcohol (PVA), polypropylene (PP), polyethylene (PE), nylon,polyester (PET), polyamide, acrylic; or inorganic materials, such asglass, ceramic, silica, metals like iron/steel, or hybrids orcombinations thereof. PVA (and modified PVA with various functionalgroups) offers very good adhesion to cement gel due to its chemistry andcan be helpful as deformable solid hollow fibers in contributing totensile strength-related properties of concrete matrix, in addition tofreeze-thaw durability.

Additionally, the solid deformable hollow elements (spheres, fibers, anyshape) can be mono-component or bi-component (sheath-core type; e.g.,core comprised of compressible foam, hydrophobic core and hydrophilicsheath). The term “hollow” refers to a deformable space of significantlylow density, such as gases or air, in general, but is not limited tobeing entirely empty. For example, a low density, easily compressiblefoam could comprise the “hollow” space, surrounded by a higher densityshell or the like. Here, the phrase “fiber shape” refers to anon-spherical shape having a high aspect ratio, preferably at leastabout 5, and is also applicable to fibers having non-circular crosssection and film-like shapes having finite thickness, length and width.

In the case of deformable solid elements comprised of inner deformablespaces and outer solid shells in contact with surrounding cement gel(for example, solid hollow elements like solid hollow spheres or solidhollow fibers), the outer shell should be flexible enough under freezingconditions to allow inner deformable spaces to be deformed and toaccommodate expansion caused by freezing water in the capillary pores ofthe cement gel. Accordingly, when the outer shell is polymeric, if theglass transition temperature of the polymer material is above ambienttemperature (e.g., above 25° C.), the outer shell is rigid even atambient conditions. Deformable solid hollow elements having rigid wallsat ambient or freezing temperatures are discussed below.

In the case of solid hollow elements having rigid walls (comprised oforganic polymeric materials, inorganic materials, metallics, hybrids orcombination therein), the rigid walls become flexible by chemical and/orphysical interactions, weaken, disintegrate and/or are eliminated duringthe curing period of the wet concrete mix, and/or during the firstfreezing event. For example, in the case of hollow glass spheres beingused as deformable solid hollow elements in a wet concrete mix per thisinvention, the crushing strength of hollow glass spheres should be wellbelow 1000 psi (i.e. the tensile strength of the cement gel), so thatthe glass walls get crushed during the first freezing event.Alternatively, since some glass formulations can be chemically attackedby the highly alkaline (pH>13) liquid phase conditions of hydratingcement, thin walls of such glass spheres are likely to be weakened,disintegrated, and/or absorbed by the hydrating cement phase duringcuring. The latter allows some margin for increasing the wall thicknessof hollow glass spheres if needed for stability of the spheres duringmixing steps (by using an appropriate glass composition thatdisintegrates during curing).

In contrast to conventional LWC, the freeze-thaw durable concrete perthis invention incorporates a volume of deformable solid hollowelements/spheres below about 2 vol. %, more preferably below about 1vol. % and most preferably below about 0.6 vol. %, relative to thevolume of said wet concrete mix or a subsequently hardened concreteformed there from, an order of magnitude lower than the amount of hollowelements used as fillers in LWC for reducing density of concrete, andthe weight fraction is a maximum of 0.2%, and as low as 0.01%, as shownin Table 1A, again an order of magnitude lower than the hollow spheresused as fillers in LWC for reducing density of concrete. The density ofconcretes of Table 1 and Table 1A is 2.3 g/cm³. The deformable solidelements can either be included in the dry concrete mix, or mixed withthe remaining components at the time of preparing a wet concrete mix.

Likewise, the deformable solid elements can have a maximum dimension ofless than about 300 micrometers, preferably less than about 200micrometers, or even less than about 100 micrometers, or even less thanabout 70 micrometers. The term “maximum dimension” as used herein refersto the diameters of the spheres or fibers used herein.

When properly formulated according to the present invention, theresulting hardened concrete will have a density above about 1.5 g/cm³,such as from about 2 g/cm³ to about 2.5 g/cm³, similar to most widelyused concretes, or even a density greater than about 2.5 g/cm³.

In the case of high density concretes, like high strength concretehaving low permeability produced using silica fumes with cement, whereinthe water-to-cement ratio is low, generally below about 40% (i.e. havinga very low level of water in excess of that needed for hydration of thecement, generally about 36%) low levels of deformable solid elements canbe added to a wet concrete mix according to this invention, as aprecautionary measure against freeze-thaw events, without noticeablyaffecting the compressive strength of the concrete, an importantconcrete performance parameter for high strength concrete.

Typically, the cement component is a Portland cement. Various known inthe art aggregates can be added to the concrete mixtures of the presentinvention for their known purposes. The type of cement, sand andaggregates used and suitable cement:sand:aggregates weight ratios dependon the desired strength properties of the hardened concrete and itsintended end-use application. The most widely usedcement:sand:aggregates weight ratio for conventional concrete is 1:2:3.The novel wet concrete mix can be comprised of Portland cement, sand,aggregates, admixtures (like retarders, accelerators, plasticizers,silica fumes, fly ash and other pozzolanic materials), water anddeformable solid elements. Said deformable solid elements of finitecontrolled dimension are uniformly mixed with dry concrete mix and/orwith wet concrete mix at the time of preparing wet concrete mix.

Likewise, various admixtures can be added to the presently disclosedfreeze-thaw durable cement mixture, for their known properties, such asretarders, accelerators, plasticizers, silica fumes, fly ash and otherpozzolanic materials, and combinations of these.

However, unlike the conventional freeze-thaw durable concretes describedabove, AEAs comprised of wetting agents, surfactants and/or foamingagents, are not used in the presently disclosed wet concrete mix for thepurpose of freeze-thaw durability. In other words, the freeze-thawconcrete mix of the present invention is or can be devoid of such AEAs,as described above.

Depending on the nature of the deformation, deformation of individualdeformable solid elements may lead to multiple deformablespaces/elements during a first freezing event and/or over periods ofrepeated freeze-thaw cycles; e.g., deformable hollow spheres with rigidwalls, as discussed above.

As an illustration, the physical dimensions of the most preferredembodiments, i.e. deformable solid hollow spheres and deformable solidhollow fibers used in the novel wet concrete mix, are estimated asfollows. Estimates are based on following assumptions:

(1) the thickness of the cement gel between adjacent hollow elements ina resulting hardened concrete matrix is a maximum of 200 micrometers,

(2) the void space of the hollow elements is at least equal to increasein volume (9%) of water freezing in the capillary pores of the cementgel of a concrete matrix,

(3) all of the hollow elements are of the same dimension, and areuniformly distributed in the cement gel,

(4) the volume of the cement gel corresponds to the volume of solidcement plus the volume of water, and

(5) the capillary pores volume of cement gel corresponds to volume ofwater in excess of 36% by weight of cement.

Assumption No. 1 is based on the fact that freezing water expanding incapillary pores can travel up to 550 micrometers in the capillary poreswithout exceeding the tensile strength of cement gel (1000 psi).Assumption No. 3, for simplicity, assumes that the hollow spheres are tobe situated as the corners of cubes in the cement gel. Using an XYZperpendicular coordinate axis, hollow fibers are situated in layers inthe cement gel, wherein each layer is situated in XY plane and whereinsaid layers are spaced along Z direction, and wherein adjacent layersare separated by 200 micrometers in Z-direction. Additionally, it isassumed that each layer of fibers are spaced end-to-end and side-by-sideby 200 micrometers and all fibers in a layer are aligned in the Xdirection and in the Y direction in adjacent layers. The Goal Seekfunction of Microsoft Excel workbook was used to make the variousestimates of IDmin and ODmax as set forth herein.

Estimated physical dimensions of hollow spheres are reported in Table 1for most widely used water-to-cement weight ratios of 0.4-0.6 and cementgel volume fraction of 0.25-0.32 in concrete matrix. The physicaldimensions of solid hollow elements shown in Table 1 are estimates basedon above described assumptions. The estimated physical dimensions andloadings (weight of solid hollow elements per unit volume of concrete)in concrete as shown in Table 1 are for illustration purposes only andare in no manner meant to be limiting as to the scope and/or spirit ofthe invention.

TABLE 1 Concrete Cement Hollow spheres in Concrete Hollow spheres Waterto gel Vol. Wt. Load- Surface Cement (vol. fraction, fraction, ing,ODmax, IDmin, Area, Void Ex. # (wt. ratio) fraction) (%) (%) (kg/m³)(μm) (μm) (mm2/mm3) Fraction 1 0.6 0.32 2 1.25 28.65 193.9 140.1 30.940.38 2 0.6 0.32 1 0.25 5.69 128.3 116.7 46.77 0.75 3 0.6 0.32 0.9 0.153.38 121.2 114.2 49.50 0.84 4 0.6 0.32 0.85 0.10 2.25 117.6 112.9 51.020.88 5 0.6 0.32 0.6 6 0.5 0.29 2 1.56 35.90 209.5 126.4 28.64 0.22 7 0.50.29 1 0.56 12.90 136.7 103.9 43.89 0.44 8 0.5 0.29 0.8 0.36 8.29 12199.1 49.59 0.55 9 0.5 0.29 0.6 0.16 3.70 104.2 93.9 57.58 0.73 10 0.50.29 0.5 0.06 1.39 95.1 91.1 63.09 0.88 11 0.5 0.29 0.4 12 0.4 0.25 21.87 43.12 229.4 91.1 26.16 0.06 13 0.4 0.25 1 0.87 20.12 147.3 73.740.73 0.13 14 0.4 0.25 0.6 0.47 10.92 111.4 66.1 53.86 0.21 15 0.4 0.250.4 0.27 6.32 90.9 61.7 66.01 0.31 16 0.4 0.25 0.2 0.07 1.71 66 56.590.91 0.63 17 0.4 0.25 0.15 0.02 0.57 58.2 54.8 103.09 0.83 18 0.4 0.250.14 0.02 0.35 56.5 54.4 106.19 0.89 19 0.4 0.25 0.13 NOTE 1: “ODmax” -outer diameter (maximum), “IDmin” - inner diameter (minimum) NOTE 2:Hardened/cured Concrete density 2.3 gms/cm³ NOTE 3: Density of solidglass 2.3 gms/cm³

Examples 1A-19A of Table 1A, below, are derived from correspondingExamples 1-19 of Table 1 by increasing the inner diameters (IDmin) ofthe hollow spheres to values corresponding to a void fraction of 0.9.Increasing the ID from a minimum value (IDmin) increases the void spacebeyond the minimum estimated value (excess void space is beneficial) andreduces the wall thickness of the hollow spheres, and thus reduces thetrue density and weight of the hollow spheres in concrete for givenvolume fraction of hollow spheres. Reduced wall thickness (thin walls)is also helpful in reducing crushing strength (compressive strength) ofhollow spheres having rigid walls to values much lower than the tensilestrength of cement gel (which is generally about 1000 psi). However, itis important that the rigid walls are not so thin/weak that they couldbe destroyed in the mixing process during preparation of a dry or wetmix, prior to the hollow spheres getting situated in place in the wetconcrete mix and prior to curing.

Additionally as to Tables 1 and 1A below, for deformable solid hollowspheres having 90% hollowness (void fraction), the estimated volumefraction of deformable elements can be as low as 0.9% and 0.14% forwater-to-cement ratios of 0.6 and 0.4 respectively, and thecorresponding maximum outer diameter of the hollow spheres is about 120micrometers and 60 micrometers, respectively. These volume fractions anddimensions of the deformable solid elements are an order of magnitudesmaller than entrained air bubbles using AEAs for freeze-thaw durableconcrete.

TABLE 1A Concrete Cement Hollow spheres in Concrete Hollow spheres Waterto gel Vol. Wt. Surface Cement (vol. fraction fraction Loading ODmax IDArea Void Ex. # (wt. ratio) fraction) (%) (%) (kg/m³) (μm) (μm)(mm²/mm³) fraction 1A 0.6 0.32 2 0.20 4.60 193.9 187.2 30.94 0.90 2A 0.60.32 1 0.10 2.30 128.3 123.9 46.77 0.90 3A 0.6 0.32 0.9 0.09 2.07 121.2114.2 49.50 0.90 4A 0.6 0.32 0.85 0.09 1.96 117.6 112.9 51.02 0.90 5A0.6 0.32 0.6 6A 0.5 0.29 2 0.20 4.60 209.5 202.3 28.64 0.90 7A 0.5 0.291 0.10 2.30 136.7 132.0 43.89 0.90 8A 0.5 0.29 0.8 0.08 1.84 121 116.849.59 0.90 9A 0.5 0.29 0.6 0.06 1.38 104.2 100.6 57.58 0.90 10A 0.5 0.290.5 0.05 1.15 95.1 91.8 63.09 0.90 11A 0.5 0.29 0.4 12A 0.4 0.25 2 0.204.60 229.4 221.5 26.16 0.90 13A 0.4 0.25 1 0.10 2.30 147.3 142.2 40.730.90 14A 0.4 0.25 0.6 0.06 1.38 111.4 107.6 53.86 0.90 15A 0.4 0.25 0.40.04 0.92 90.9 87.8 66.01 0.90 16A 0.4 0.25 0.2 0.02 0.46 66 63.7 90.910.90 17A 0.4 0.25 0.15 0.02 0.35 58.2 56.2 103.09 0.90 18A 0.4 0.25 0.140.01 0.32 56.5 54.6 106.19 0.90 19A 0.4 0.25 0.13

Each Example of Tables 1 and 1A produces a concrete matrix having athickness of cement gel between hollow spheres no greater than about 200micrometers and provides needed space for accommodating expansion offreezing water in the capillary pores without allowing a pressurebuild-up in the capillary pores exceeding 1000 psi (the tensile strengthof cement gel), and thus offers a recipe for producing a novelfreeze-thaw durable concrete according to the present invention. Asshown in Tables 1 and 1A, the volume fraction of deformable hollowspheres can be as low as 0.9 for a water-to-cement weight ratio 0.6(high), or as low as 0.5 for a water-to-cement ratio 0.5 (moderate), oreven as low as 0.15 for a water-to-cement ratio 0.4 (low). These volumefractions of deformable hollow spheres in concrete are an order ofmagnitude smaller compared to using 4-8% entrained air by volume to formcapillary pores in concrete for freeze-thaw durability. The maximumdimension of deformable hollow spheres can be as low as 70 micrometers,which is an order of magnitude smaller than air bubbles as large as 1000micrometers in case of entrained air using AEAs in wet concrete mix,discussed above.

For each Example of Tables 1 and 1A, deformable hollow spheres ofsmaller dimensions than ODmax (same void fraction) when used at the samevolume fraction lowers the thickness of the cement gel between thehollow spheres to below about 200 micrometers, which is likely to bevery helpful in making the concrete more freeze-thaw durable.

The estimated value of outer diameter of the deformable hollow spheresreported in Table 1 is based on assumption of all spheres are of samedimension and uniformly distributed in cement gel. The required loadingcan be more than estimated if the hollow glass spheres are not uniformlydistributed in wet concrete mix. As discussed above, glass having anappropriate chemical composition can be chemically attacked by thehighly alkaline (pH>13) liquid phase of hydrating cement, and hence 3M™hollow glass microspheres that have heavier wall thickness (e.g., StylesS32, S35, K37 where initial crushing strength is above 1000 psi, voidfraction about 85%, 50% of hollow spheres less than about 50 micrometersin dimension) that are still within estimated physical dimensions asshown in Table 1A can be made suitable with the above described properdecomposable glass (in case heavier wall thickness is desired forstability of hollow glass spheres during mixing). Deformable hollowspherical elements smaller than 10 micrometers may be desired to bescreened out in view of the desired void and wall thicknesses forsuitability of hollow spheres per this invention.

As illustrated in Tables 1 and 1A, the volume fraction of hollow glassspheres in the novel freeze-thaw durable concrete is from about 0.1-1%,as compared to the volume fraction of entrained air of about 4-8% inconventional freeze-thaw durable concrete. In addition, the dimensionsof hollow glass spheres is an order of magnitude smaller thancorresponding air bubbles of entrained air-based conventionalfreeze-thaw durable concrete. The maximum dimensions of the hollow glassmicrospheres can be controlled to be about 200 micrometers, as comparedto air bubbles as large as 1000 micrometers for entrained air.

Thus, resulting novel concrete matrix, in addition to having goodfreeze-thaw durability, can also exhibit enhanced compressive strengthand other strength-related properties of a concrete matrix (likeabrasion resistance, impact strength, toughness) that are currentlynegatively affected by the presence of large entrained air bubbles inlarge quantities. If the novel concrete mix is also formulated tocontain conventional reinforcing fibers for structural value in aconcrete matrix, the full potential of reinforcing fibers can berealized due to absence of large quantities of entrained air in the formof large air bubbles, resulting in a novel freeze-thaw durable,fiber-reinforced concrete matrix, having step increases in key strengthproperties like abrasion resistance, impact strength, toughness, ascompared to conventional fiber reinforced concrete containing 4-8% byvolume entrained air for freeze-thaw durability. This is of significantvalue in applications like infrastructures, such as highways.

Table 2 illustrates various attributes of hollow fibers suitable forformulating the freeze-thaw durable concrete of the present invention.Considering current manufacturing capabilities for hollow fibersproduction, it is preferred to keep the void fraction of hollow fibersto less than 60%, preferably less 40%, more preferably less than 30%,and fiber dimensions (outer diameter) greater than 10 micrometers (+1denier for resins like PP, PET, PVA). For cost effectiveness, it ispreferred to keep the physical dimensions of hollow fibers for use inthis invention within current manufacturing practices for hollow fibers,and the required hollow fiber loading in concrete as low as feasible.

TABLE 2 Concrete Hollow fibers in Concrete Hollow fibers Water to Cementgel Vol. PP hollow PVA hollow Surface Cement (vol. fraction, fibers,fibers ODmax IDmin Length Area Void Ex. # (wt. ratio) fraction) (%)(kg/m³) (kg/m³) (μm) (μm) (cm) (mm²/mm³) fraction 20 0.40 0.25 0.28 1.402.00 27.30 18.30 2.00 146.52 0.45 21 0.40 0.25 0.40 2.49 3.56 33.5018.80 2.00 119.40 0.31 22 0.40 0.25 0.50 3.40 4.86 38.20 19.20 2.00104.71 0.25 23 0.45 0.27 0.60 2.88 4.12 40.90 28.10 2.00 97.80 0.47 240.45 0.27 0.80 4.70 6.72 48.90 29.10 2.00 81.80 0.35 25 0.45 0.27 1.006.52 9.32 56.20 29.90 2.00 71.17 0.28 26 0.50 0.29 1.50 9.64 13.77 70.3038.10 2.00 56.90 0.29 NOTE 1: “ODmax” - outer diameter (maximum),“IDmin” - inner diameter (minimum) NOTE 2: Hardened/cured Concretedensity 2.3 gms/cm³ NOTE 3: Density of PP (Polypropylene) 0.91 gms/cm³;Density of PVA 1.3 gms/cm³

The novel concrete mix of the present application may comprise mixturesof various different deformable elements (shape, dimension, make-up).Minor variations like the use of very low levels of entrained air, suchas less than about 1 vol %, along with the deformable elements in wetconcrete mix per this invention is within the spirit/scope of thisinvention. As discussed above, the deformable solid elements can beadded to a dry concrete mix and/or a wet concrete mix. Dry concrete mixcomposition is same as wet concrete mix excluding water if deformablesolid elements are added to wet concrete mix. Since the volume ofresulting hardened concrete is almost same as wet concrete mix,parameters like volume fraction and the spacing of solid deformableelements in wet concrete mixes, are essentially equivalent for hardenedconcrete as well.

Unless otherwise specified, the meanings of terms used herein shall taketheir ordinary meaning in the art. In addition, all patents and patentapplications, test procedures (such as ASTM methods or the like), andother documents cited herein are fully incorporated by reference to theextent such disclosure is not inconsistent with this invention and forall jurisdictions in which such incorporation is permitted. Also, whennumerical lower limits and numerical upper limits are listed herein,ranges from any lower limit to any upper limit are contemplated. Notefurther that Trade Names used herein are indicated by a ™ symbol or ®symbol, indicating that the names may be protected by certain trademarkrights, e.g., they may be registered trademarks in variousjurisdictions.

The invention has been described above with reference to numerousembodiments and specific examples. Many variations will suggestthemselves to those skilled in this art in light of the above detaileddescription. All such obvious variations are within the full intendedscope of the appended claims.

I claim:
 1. A concrete mix for producing a freeze-thaw durable concrete,comprising cement, inorganic, hollow, spherical, deformable solidelements, which elements are deformable by crushing at a pressure of<1000 psi, and optionally water, wherein said inorganic deformable solidelements are either included in said concrete mix in a dry state, ormixed with said cement at the time of preparing a wet concrete mix, arepresent at a level of less than or equal to about 2 vol % and 1.56 wt %relative to the volume of said wet concrete mix or a green concrete or asubsequently hardened concrete formed therefrom, or less than 1.7 wt %in the dry state based on the total weight of all dry materials, andhave a maximum dimension of less than about 300 micrometers.
 2. Theconcrete mix of claim 1, further comprising sand, aggregates,admixtures, or reinforcing fibers or combinations thereof.
 3. Theconcrete mix of claim 1, wherein a total deformable space of saidinorganic deformable solid elements is at least 9% of capillary poresvolume, wherein the capillary pores volume is equal to the volume ofwater in excess of 36% by weight of cement in wet concrete mix.
 4. Theconcrete mix of claim 1, wherein the total volume of said deformableelements is sufficient to accommodate expansion caused by water freezingin capillary pores of said subsequently hardened concrete during afreezing event.
 5. The concrete mix of claim 1, wherein water is presentat a water-to-cement weight ratio in said wet concrete mix of at leastabout 26%.
 6. The concrete mix of claim 5, wherein the water-to-cementweight ratio in said wet concrete mix is 26 to 40%.
 7. The concrete mixof claim 5, wherein water is present at a water-to-cement weight ratioin said wet concrete mix from about 40% to about 60%.
 8. The concretemix of claim 2, wherein said admixtures include one or more ofretarders, accelerators, plasticizers, fillers, silica fumes, fly ash,other pozzolanic materials or combinations thereof.
 9. The concrete mixof claim 1, wherein said inorganic deformable solid elements have arigid shell.
 10. The concrete mix of claim 9, wherein said inorganicdeformable solid elements are made of materials selected from the groupof glass, ceramic and silica and combinations thereof.
 11. The concretemix of claim 10, wherein said inorganic deformable solid elements arecenospheres.
 12. The concrete mix of claim 1, wherein said inorganicdeformable solid elements are glass spheres having rigid shells and acrush strength of <1000 psi.
 13. The concrete mix of claim 1, whereinthe deformable solid elements have diameters from about 10 micrometersto about 200 micrometers.